In the 3GPP (Third Generation Partnership Project) that defines a standard for mobile network systems, EPC (Evolved Packet Core) is specified as the next generation mobile network system. The EPC is a network system that accommodates an LTE (Long Term Evolution) access network in addition to wireless access networks called the second generation and the third generation.
Downlink data packet processing in the EPC is described hereinafter. In Non-Patent Literature 1, the downlink data packet processing in the case where a UE (User Equipment) representing a user terminal is in the idle mode is disclosed. The state where the UE is in the idle mode is the state where the UE is not connected with the EPC. In other words, a wireless bearer between the UE and the EPC is disconnected in this state. In such a case, when a downlink data packet is made to the UE, an SGW (Serving Gateway), which is a bearer management device, sends a downlink data notification message to an MME (Mobility Management Entity) or an SGSN (Serving GPRS Support Node) that manages the UE. Receiving the downlink data notification message, the MME or SGSN sends a Paging message to an eNB (evolved Node B) or a NodeB, which is a base station, in order to page the UE. Receiving the Paging message, the eNB or NodeB performs Paging processing on a plurality of UE located in the area managed by itself. The UE to be paged sends a response message to the eNB or NodeB and connects to the EPC.
In this manner, even when a downlink data packet is made to the UE in the idle mode which is not connected with the EPC, it is possible to perform data communication with the UE by paging the UE from the network side.
Further, when ISR is active, or activated in other words, in UE, SGSN, MME and S-GW, the S-GW sends a downlink data notification message to the MME and the SGSN. In this case, both of the MME and the SGSN perform Paging processing on the UE. Thus, when ISR is active, Paging processing is performed in both of the 2G/3G wireless access network and the LTE access network.
The state where ISR is active is the state where the UE is in ATTACH to the two devices, the SGSN and the MME, at the same time. Once ISR is active, mobility management of the UE is performed without sending TAU (Tracking Area Update) and RAU (Routing Area Update) from the UE even when the UE moves between the 2G/3G wireless access network and the LTE access network.